1. Technical Field
This invention relates to riser elbows for an inboard internal combustion marine engine, and more particularly, to such riser elbows having an exhaust gas conduit section formed of a fiber reinforced ceramic matrix composite (FRCMC) material and methods for making these elbows.
2. Background Art
The exhaust from an inboard internal combustion marine engine used to power recreational and small commercial boats, as well as larger vessels, is typically cooled to prevent excessive heat buildup in the exhaust system. This cooling is necessary to protect the structure of the boat from the heat. For example, hot exhaust gases which impinge the fiberglass hull employed in many recreational boats can degrade or even burn the fiberglass. The same is true for exhaust system component heated by these gases which are adjacent the boat's hull. In addition, the temperature of the exhaust system components must be kept below the flash point of gasoline for safety reasons. U.S. Coast Guard requirements dictate that exhaust system components not exceed a prescribed touch temperature. Typically, the touch temperature of these components must be below 210 degrees Fahrenheit, or 185 degrees Fahrenheit in some cases. As a result many of the exhaust systems components of a marine engine must be cooled as well, typically using a water jacket structure of some type. In addition, the exhaust gas itself is usually cooled by injecting water into the exhaust gas stream.
FIG. 1 depicts in simplified form a typical exhaust system for a marine engine. Exhaust gas exits the exhaust manifold 12 of the engine into the exhaust gas conduit 14 of a riser elbow 16. In addition, cooling water, which is typically drawn from the surrounding body of water to cool the engine and exhaust system components, exits the a water jacket associated with the manifold 12 into the cooling water channel 18 of the riser elbow 16. The cooling water channel 18 is formed by an external sleeve 20 surrounding the exhaust gas conduit 14. The cooling water keeps the riser elbow 16 below the required touch temperature despite the exhaust gas flowing through the conduit 14. The end of the external sleeve 20 of the riser elbow is coupled to the inlet side of a flexible connector conduit 22, which is typically made of rubber tubing. The end of the exhaust gas conduit 14 opens up into the interior of the connector conduit 22. In this way, the cooling water and exhaust gas are mixed, thereby cooling the gas. The outlet side of the connector conduit 22 is coupled to the inlet of an exhaust pipe 24. This exhaust pipe 24 is typically made from stainless steel to resist the corrosive effects of the exhaust gas and cooling water mixture flowing through it. Although, not shown in FIG. 1, the exhaust gas and cooling water mixture eventually exits the boat via the outlet of the exhaust pipe 24. It is noted that no water jacket structure is required for the connector conduit 22 and exhaust pipe 24 as the mixing of the cooling water with the exhaust ensures these components remain below the required touch temperature.
The riser elbow 16 is required in the exhaust system of a marine engine to prevent the cooling water used to cool the exhaust from finding its way back into the engine. Essentially, the riser elbow 16 constitutes a high point in the exhaust system upstream of the connector conduit 22 where the cooling water is injected into the exhaust gas. This high point in the system prevents the cooling water from falling back into the engine.
The structure of the riser elbow creates some unique problems. As can be seen in FIG. 1, the hot exhaust gases make a very abrupt turn within the conduit 14 of the riser elbow. This results in direct impingement of a hot, corrosive gas on the interior surface of the conduit 14. In addition, the cooling water flowing through the cooling water channel may have corrosive effects of its own. For ocean-going vessels, this cooling water is typically raw salt water pumped directly into the engine. And finally, when the cooling water and exhaust gas is mixed in the connector conduit 22, hot, corrosive steam is produce which contacts the outlet side of the riser elbow. The aforementioned factors combine to present an extremely corrosive environment which the riser elbow must survive. Riser elbows are typically made from either cast iron or a thin-wall stainless steel casting. In the case of a cast iron elbow, the corrosive effects cause the cast iron to decay very quickly. A cast iron riser elbow may not survive more than 300-400 hours total use before the outlet end corrodes away. Additionally, a cast iron riser elbow typically has thick walls and has a considerable weight. This results in an increase in the weight of the vessel and reduces the efficiency of the engine. The use of a thin-walled stainless steel casting resolves the weight problem. In addition, stainless steel corrodes at a much slower rate than cast iron and so lessens the corrosion problem. Therefore, this type of riser elbow does not have to be replaced as often as a cast iron riser elbow. However, the thin-walled stainless steel riser elbows are considerably more expensive than the cast iron versions. Thus, the cost is high no matter which version is employed. The cast iron because of the need to replace it often, and the stainless steel because of its high initial cost.
Riser elbows made from stainless steel also present another problem. Whereas, a typical cast iron exhaust manifold and attached cast iron riser elbow expand at nearly the same rate as the engine heats up, a stainless steel riser elbow does not. A thin-walled stainless steel riser elbow expands at a significantly different rate than a cast iron exhaust manifold. This disparity in the thermal expansion rates of these two exhaust system components can result in the seal between the two being compromised resulting in exhaust gas and/or cooling water leaks.
Another problem concerning existing cast iron and stainless steel riser elbows is their high thermal conductivity. Having a high thermal conductivity results in heat from the exhaust being readily transferred through the exhaust gas conduit to the cooling water. Thus, to ensure the riser elbow remains below the prescribed touch temperature, it is necessary to flow sufficient quantities of cooling fluid to dissipate the heat. This in turn can increase the cooling water requirements over that which is required to cool the exhaust gas in the downstream exhaust system components, thereby requiring bigger water jackets and/or cooling water flow rates. These increased cooling requirements can lead to the need for larger engine components, thereby increasing the weight and lowering the efficiency of the engine.
Accordingly, there is a need for a light-weight, low cost, thermally insulative riser elbow which can withstand the corrosive environment of a marine engine exhaust system, and which has a thermal expansion coefficient closer to that of the engine's cast iron exhaust manifold.
Wherefore, it is an object of the present invention to provide a riser elbow for a marine engine which is lighter in weight than existing cast iron riser elbow, so as to reduce the overall weight of vessel and increase the efficiency of the engine.
Wherefore, it is another object of the present invention to provide a riser elbow for a marine engine which exhibits a low thermal conductivity so as to retain more heat within the exhaust gas flowing therethrough, thereby reducing the cooling requirements of the elbow while still maintaining the touch temperature below a prescribed level.
Wherefore, it is yet another object of the present invention to provide a riser elbow for a marine engine which can withstand long term exposure to the corrosive environment of the engine's exhaust system while at the same time having lower cost than comparable stainless steel riser elbows.
Wherefore, it is still another object of the present invention to provide a riser elbow for a marine engine which exhibits a thermal expansion coefficient closer to that of the engine's exhaust manifold than current stainless steel riser elbows.